Apparatus and method for calibrating a surgical knife

ABSTRACT

An apparatus and method for zeroing, calibrating and setting a surgical knife. The surgical knife includes a blade that has a tip which can move relative to a footplate that extends from the body of the knife. The apparatus includes a calibration housing that is coupled to a surgical microscope. The calibration housing has a transparent sleeve coupled to a holder, which are both adapted to receive and support the surgical knife. The calibration housing is constructed so that the knife and sleeve are in the line of sight of the microscope. The knife typically has a sensor that can sense the position of the blade tip relative to the footplate. The sensor provides an output signal to a visual display which converts the signal to a visual readout. The visual display also has an input device that allows the surgeon to zero the visual display. To calibrate the knife, the surgeon inserts the knife into the calibration housing until the footplate is parallel with a marking on the sleeve. The surgeon then adjust the knife until the tip of the blade is parallel with the marking and the footplate. When the three members are parallel the surgeon &#34; zeros&#34; the visual display. Movement of the blade within the sleeve can also be used to calibrate the visual display and set the knife.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for zeroing,calibrating and setting a surgical knife.

2. Description of Related Art

There presently exist a surgical procedure for correcting defects in thehuman eye such as myopia and astigmatism, which is commonly known asradial keratotomy. Radial keratotomy includes cutting a series ofradial, non-penetrating incisions on the outer periphery of the cornea.The incisions alter the shape of the cornea and correct the deficienciesin the eye.

The incisions are typically performed by a knife that is held andmanipulated by the surgeon. Surgical knives used for radial keratotomyhave a blade which can move relative to a footplate that extends fromthe body of the knife. The incisions are formed by placing the footplateon the surface of the cornea and moving the knife across a portion ofthe eye. The depth of the cut is therefore determined by the relativedistance between the footplate and the tip of the blade.

The knife typically has a thimble that allows the surgeon to move theblade relative to the footplate. The length of the blade is set byrotating the knife a predetermined number of turns. A typical radialincision depth is between 580-620 microns with a required accuracy ofplus or minus 5 microns. It is therefore important to establish a highdegree of accuracy when setting the blade of the knife.

U.S. Pat. No. 4,750,489 issued to Berkman et al, discloses a surgicalknife and a method for setting the blade relative to the footplate ofthe knife. Incorporated into the body of the Berkman knife is a linearvariable differential transformer ("LVDT") which senses the position ofthe blade relative to the footplate. The LVDT is coupled to a visualdisplay which provides a readout that indicates the distance between thetip of the blade and the footplate.

To obtain an accurate readout, the blade must be initially aligned or"zeroed" with the footplate. Zeroing the Berkman system is performed byinserting the knife into a zeroing sleeve. The zeroing sleeve has areflective membrane that reflects a pattern of light to a plurality oflight sensors. The knife is inserted into the sleeve until the footplateis in contact with the reflective membrane. The knife is then adjusteduntil the blade advances to a point where the tip deflects thereflective membrane. Deflection of the membrane disturbs the lightpattern which is sensed by the sensors and interpreted by the visualdisplay as the zero position of the blade. The Berkman system forcalibrating the knife has been found to be somewhat unreliable to useand expensive to produce.

U.S. Pat. No. 4,662,075 issued to Mastel et al, discloses another methodof zeroing and setting a surgical knife that uses an apparatus typicallyreferred to as a micron table. The micron table has a microscope coupledto an X-Y table. The lens of the microscope has a cross shaped reticlewhich provides a pair of reference lines for the user. The knife isplaced on the X-Y table and the table is adjusted until the footplateand blade are centered within the reticle. The knife thimble is thenrotated until the tip of the blade is zeroed with the footplate of theknife. After the blade is zeroed, the X-Y table is adjusted to move thefootplate and blade a predetermined distance from the center of thereticle. The knife thimble is again rotated so that the blade is movedback to the center of the reticle. The knife is then transferred to asterile operating field.

During the transportation of the knife, the tip may move to a differentlocation, resulting in an inaccurate blade setting. When this occurs theknife must be returned to the micron table and reset. Additionally,while performing surgery it may become apparent to the surgeon that theblade setting is either too short or too long. Resetting the knife againrequires a trip back to the micron table. The micron table is typicallylocated separate from the operating field, wherein the knife may requireresterilization every time the knife is calibrated and set. Such aprocedure wastes valuable operating time. It would therefore bedesirable to have a system for accurately zeroing and setting a surgicalknife at the operating site.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for zeroing,calibrating and setting a surgical knife. The surgical knife includes ablade that has a tip which can move relative to a footplate that extendsfrom the body of the knife. The knife also has a thimble which allows asurgeon to move the blade relative to the footplate. The blade ispreferably coupled to the thimble by differential threads which providesmall increments of translational movement in response to relativelylarge angular displacements of the thimble. The differential threadsalso prevent the blade from drifting after the tip is set. The apparatusincludes a calibration housing that is coupled to a surgical microscopetypically used to perform a surgical procedure such as radialkeratotomy. The calibration housing has a transparent sleeve coupled toa holder, which are both adapted to receive and support the surgicalknife. The sleeve is preferably flat to reduce the parallax of theoptical system.

The knife is inserted into the calibration housing until the footplateis parallel with a first marking on the sleeve. The calibration housingis constructed so that the knife and sleeve are in the line of sight ofthe microscope. The present invention therefore allows the surgeon toaccurately view the footplate, blade tip and the first marking of thesleeve.

The knife typically has a sensor that can sense the position of theblade tip relative to the footplate. The sensor sends an output signalto a visual display which converts the signal to a visual readout. Thereadout provides the distance between the tip of the blade and thefootplate. The visual display also has an input device that allows thesurgeon to zero the visual display.

To calibrate the knife, the surgeon inserts the knife into thecalibration housing until the footplate is parallel with the firstmarking on the sleeve. The surgeon then adjust the knife until the tipof the blade is parallel with the first marking and the footplate. Whenthe three members are parallel the surgeon "zeroes" the visual display.The sleeve may also have a second marking located a predetermineddistance from the first marking. The accuracy of the visual display canbe determined by moving the tip of the blade to the second marking andthen looking at the readout to see if the display is providing thecorrect value.

The blade of the knife can be set by rotating the thimble and moving thetip until the visual readout provides the desired value. The sleeve mayalso have a scale to provide a visual indication of the distance betweenthe tip of the blade and the footplate of the knife. The scale can beused with or without the sensor and visual display. The calibrationhousing is pivotally connected to the microscope, so that the surgeoncan remove the knife, move the housing out of the line of sight andbegin the surgical procedure.

Therefore it is an object of the present invention to provide a systemfor accurately zeroing a surgical knife at the operating site.

It is also an object of the present invention to provide a system foraccurately calibrating a surgical knife at the operating site.

It is also an object of the present invention to provide a system thatallows the surgeon to visually set the blade length of a surgical knifeat the operating site.

It is also an object of the present invention to provide a surgicalknife that can be accurately set and has a blade that does not drift.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become morereadily apparent to those ordinarily skilled in the art after reviewingthe following detailed description and accompanying drawings, wherein:

FIG. 1 is a perspective view of a system of the present invention;

FIG. 2 is a perspective view of a surgical knife connected to a visualdisplay system;

FIG. 3 is a perspective view of a calibration housing of the system;

FIG. 4 is a top view of a knife inserted into the calibration housing;

FIG. 4a is an enlarged top view showing a scale formed in the sleeve ofFIG. 4;

FIG. 5 is a cross-sectional view taken at line 5--5 of FIG. 4;

FIG. 5b is a side view of FIG. 4a;

FIG. 6 is front perspective view of a calibration sleeve;

FIG. 7 is a rear perspective view of the sleeve of FIG. 6;

FIG. 8 is a cross-sectional view taken at line 8--8 of FIG. 4;

FIG. 9 is a cross-sectional view of taken at line 9--9 of FIG. 4;

FIG. 10 is a perspective view of an alternate embodiment of thecalibration housing showing a second sleeve within a first sleeve;

FIG. 11 is a perspective view of an alternate embodiment of thecalibration housing showing a gear assembly that can move the blade ofthe knife;

FIG. 12 is a schematic of an optical system of the present invention;

FIG. 13 is a side view of an alternate embodiment of the system;

FIG. 14 is a cross-sectional view of a knife;

FIG. 15 is a side view of an alternate embodiment of the knife of FIG.14.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings more particularly by reference numbers, FIG. 1shows a system 10 of the present invention. The system 10 is used tozero, calibrate and set a surgical knife. The knife is typically used toperform surgical procedures such as radial keratotomy, although it is tobe understood that the knife and system can be used for other surgicalprocedures. The system 10 is located in a sterile environment,preferably at the operating site. It is to be understood that one of themain advantages of the present system is the ability of the surgeon tozero, calibrate and set the knife in a sterile room and at the site ofthe surgery. Although the system is described as being operated by asurgeon, it is to be understood that the present invention may bepracticed by other qualified personnel such as a nurse.

In the preferred embodiment, the system 10 has a calibration housing 12coupled to a microscope 14. The microscope 14 has a pair of eyepieces 16that allow the surgeon to view magnified objects as is known in the art.The microscope 14 is typically suspended from the ceiling and locatedabove the operating table. The system 10 also includes a surgical knife18 that is connected to a visual display system 20.

As shown in FIG. 2, the surgical knife 18 has a blade 22 that can moverelative to the body 24 of the knife 18. The knife 18 includes a sensor26 that can determine the relative position of the blade tip 28 to afootplate 30 which extends from an end of the body 24. The knife has athimble 32 coupled to the blade 22. Rotation of the thimble 32 moves theblade 22 relative to the footplate 30. The thimble 32 allows the surgeonto manually adjust the blade 22 of the knife.

The sensor 26 provides an output signal to the visual display system 20,which receives the output signal and provides a readout that indicatesthe distance from the blade tip 28 to the footplate 30. The visualdisplay system 20 may include a main display unit 34 and a remotedisplay unit 36. The main unit 34 typically contains the electronichardware of the display system 20 and a main readout display 38 thatprovides the distance between the tip of the blade and the footplate 30.The remote unit 36 is connected to the main unit 34 through a cable 39which allows the unit 36 to be in close proximity to the surgeon. Theremote unit 36 has a remote readout display 40 that provides the sameblade distance readout as the main unit 34. The visual display system 20also has a control button 41 which allows the surgeon to set the visualdisplay to zero. The control button 41 is preferably located on theremote unit 36, but may also be positioned near the microscope 14 andcalibration housing 12.

FIG. 3 shows a calibration housing 12 which includes a first sleeve 42coupled to a holder 44. The holder 44 has a first inner cavity thatallows the knife 18 to extend therethrough. The sleeve 42 has a secondinner cavity 48 that generally conforms to the outline of the knife 18.As shown in FIGS. 4a and 5b, the sleeve 42 has a first marking 50 thatprovides a reference point and at least one second marking 52 located apredetermined distance from the first marking 50. The sleeve 42 may alsohave additional markings to create a scale. The sleeve 42 is preferablyconstructed entirely from a transparent plastic such as acrylic.

As shown in FIG. 1, the calibration housing 12 is attached to an arm 54that is connected the microscope 14. The arm 54 is typically anextension of the holder 44. The arm 54 and calibration housing 12 areconstructed so that the sleeve 42 is in the line of sight with themicroscope 14, when the arm 54 is in the "extended" position. Thisposition allows the surgeon to accurately view the sleeve 42 and theblade 22 of the knife. The arm 54 is adapted to move the sleeve 42 outof the line of sight of the microscope 14, when the calibration housing12 is moved into a retracted position. The arm 54 may have detents (notshown) to lock the arm 54 into the extended and retracted positions.

To calibrate the surgical knife 18, the arm 54 is moved into an extendedset position and the knife 18 is inserted into the calibration housing12. As shown in FIGS. 4 and 5, the knife 18 is inserted into the housing12 until the footplate 30 is parallel with the first marking 50. Thesleeve 42 has clamping walls 60 that capture the body 24 and hold thesame in place. The tolerances between the clamping area 60 and the firstmarking 50 are minimal, so that the footplate 30 is aligned with themarking 50 when the knife is fully inserted into the housing 12.

The surgeon then rotates the thimble 32 of the knife and moves the blade22 until the tip 28 is parallel with the first marking 50 and thefootplate 30. Once the blade tip 28 is zeroed, the surgeon pushes thecontrol button 41 and zeros the visual display. After the blade tip 28is zeroed, the surgeon may remove the knife 18 and set the blade 22, byturning the thimble 32 until the readout indicates that the tip 28 hasreached the desired distance from the footplate 30. Alternately, thesurgeon may set the knife 18 within the calibration housing 12 andvisually verify the blade length with the scale on the sleeve 42. It isto be understood that the scale can be used to set a knife that does nothave a sensor 26 and a visual display system 20.

After zeroing the blade tip, the surgeon can determine the accuracy ofthe knife sensor 26 and visual display system 20 by moving the blade tip22 from the first marking 50 to the second marking 52 and viewing thereadout display. The readout should correlate to the distance betweenthe first and second markings. For example, if the distance betweenmarkings is 200 microns, the visual display should read 200 microns whenthe blade is moved to the second marking. A different readout providesan indication to the surgeon that the sensor and/or visual displaysystem is in error. The display system 20 may have a second calibrationbutton 43 which allows the surgeon to recalibrate the knife sensor 26.In the example above, if the display does not readout "200", the surgeondepresses the second button 43. The system 20 is then recalibrated sothat movement of the blade will be accurately represented by the system.

FIGS. 6-9 show a preferred embodiment of the sleeve 42. The sleeve 42has a first annular flange 61 that is adapted to come into contact withthe holder 44. The sleeve 42 also has a second annular flange 62 that isreceived by an annular groove (not shown) in the holder 44. The flange62 and groove are constructed to align the sleeve 42 with the holder 44and prevent relative movement between the members. The sleeve 42 alsohas a pair of slots 64 that are adapted to receive a pair of keys 65that extend from the holder 44. The keys 65 are adapted to slide intothe slots 64 and attach the sleeve 42 to the holder 44. The key/slotarrangement also prevents relative rotation between the two members.

The portion of the sleeve 42 containing the markings 50 and 52preferably has a rectangular cross-section. Such a construction providesa flat surface which is in line with the microscope. The flat surfacegreatly reduces any parallax in the optical system. The sleeve 42 alsohas a pair of oblique walls 68 that provide a stop for the footplate 30of the knife 18. The oblique walls 68 also capture the footplate 30 andprevent the knife from rotating relative to the sleeve 42. As analternate embodiment, the markings 50 and 52 may be located on theoblique walls 68 and the sleeve 42 may be rotated 90° relative to theholder 44. The markings on the oblique walls appear to the viewer to becloser together than the actual distance that separates the lines. Suchan arrangement may decrease the cost of manufacturing the sleeve and/orreduce the scale of the reticle. As a further alternative, the sleeve 42could be constructed to have a circular cross-section, wherein themarkings 50 and 52 are a plurality of annular rings extending around thecircumference of the sleeve 42.

As shown in FIG. 5b, each marking preferably has a companion markinglocated on the opposite side of the sleeve 42. The dual markings allowthe surgeon to determine when the microscope 14 is perpendicular withthe marking in interest. For example, when zeroing the knife 18, thesurgeon should only see the first marking 50 on the top surface 70 ofthe sleeve 42. If the bottom first marking is visible, then themicroscope is not perpendicular with the sleeve 42. The microscope 14 istypically initially assembled so that first and second markings arealigned.

FIG. 10 shows an alternate embodiment of the calibration housing whereina second sleeve 72 is inserted into the inner cavity 48' of a firstsleeve 42'. The second sleeve 72 prevents the first sleeve 42' fromcoming in contacting with and possibly contaminating the blade 22 of theknife. The second sleeve 72 is typically replaced after each surgicalprocedure. Incorporation of the second sleeve 72 is particularly usefulwhen it is not cost effective to continuously replace the first sleeve42 and the fine markings.

FIG. 11 shows another alternate feature of the present invention. Theassembly may include an additional sleeve 74 that moves the blade of theknife. The sleeve 74 may have a first set of internal teeth 76 that arecoupled to external teeth (not shown) on the knife thimble 32, and asecond set of internal teeth 78 that are coupled to a motor/gearassembly 80. The second teeth 78 and gear assembly 80 are preferablyarranged as an eccentric gear assembly with the gear assembly containingthe planetary gear. In operation, the motor/gear assembly rotates thesleeve 74, which rotates the thimble 32 and moves the blade 28. Theelectric motor may be connected to a computer which automatically movesand sets the blade of the knife. The depth of the incision during aradial keratotomy procedure is determined by the thickness of thecornea. The cornea is typically measured by a pachymeter which can storethe dimensions in a data base. The computer can then utilize themeasurements to automatically drive the motor/gear assembly 80 and setthe blade length of the knife.

FIG. 12 shows an optical system of the present invention. During eyesurgery, the microscope 14 is typically set to magnify an object between18X-20X. Calibrating and setting the knife typically requires a largermagnification. To increase the power of the optical system, a focusinglens system 84 is inserted between the microscope and the sleeve 42. Thelens system 84 is preferably constructed to increase the power of theoptical system in the range of 50X-100X. The lens system 84 allows thesurgeon to accurately view the relationship between the blade tip andthe fine markings on the sleeve. Between the lens system 84 and thesleeve 42 is a prism 86 or other refraction device for bending the lightdirectly into one of the eyepieces. To reduce parallax, the other eyepiece is directed to another object by a mirror 88. The other object ispreferably the readout display 38 of the visual display system. The dualoptic system allows the surgeon to view the blade 22 and sleeve 42 withone eye and look at the visual readout with the other eye, withoutleaving the microscope 14. As shown in FIG. 1, the lens system 84, prism86 and mirror 88 are preferably attached to a plate 90 that extends fromthe arm 54. In the preferred embodiment, a light source is locatedbeneath the sleeve 42 opposite the lens system 84 to fully illuminatethe knife 18 and sleeve 42. The light source may include a filter thatwill only allow a predetermined wavelength(s) of light to be directedonto the sleeve 42. In the preferred embodiment, the sleeve 42 isilluminated with either amber or green light.

As an alternate embodiment, the optical system can be constructedwithout the mirror 88, wherein the light is bent directly into botheyepieces. Light from the readout display can be directed into theeyepieces so that the surgeon can see the readout while viewing theblade and sleeve. Such an optical system is commonly referred to as a"heads up" display.

FIG. 13 shows another alternate embodiment, wherein the calibrationhousing 12 and lens system 84 can move along a "dove tail" table 92between the retracted and extended positions.

FIG. 14 shows a preferred embodiment of a knife 100. The knife 100 has abody 102 which has an inner cavity 104. The knife 100 also has a blade106 that is attached to a first shaft 108 by a coupling block 110 and aplurality of screws 112. The blade 106 can move relative to a footplate114 that extends from a footplate housing 116 which screws onto the endof the body 102 of the knife. The housing 116 may contain a bearing 118which supports and aligns the shaft 108.

The first shaft 108 has a threaded portion 116 that engages an internalthread 118 of a second shaft 120. The second shaft 120 has an externalthread 122 that engages an internal thread 124 of the body 102. Thesecond shaft 120 also has a pair of annular flanges 126 separated by acollar 128. The second shaft 120 is coupled to a thimble 130 by a setscrews 132 which engage the collar 128.

The blade 106 can be moved by rotating the thimble 130. Rotation of thethimble 130 rotates the second shaft 120 relative to the body 102 andthe first shaft 108 relative to the second shaft 120. The translation ofthe shafts 108 and 120 are in opposite direction such that the actualmovement of the blade 106 is equal to the difference in the movements ofthe shafts. The differential movement of the shafts allows the user toinduce small increment movements of the blade 106 with relatively largeangular displacements of the thimble 130. Such a relationship allows theuser to more accurately move the blade in the small increments typicallyrequired to calibrate and set the knife. In the preferred embodiment,the turn ratio of the threads 122/124 to the threads 116/118 isproportioned to allow a blade extension of between 100-500 microns foreach thimble revolution. The opposing motion of the shafts insuresconstant contact between the threads of the first shaft 108 and theinternal threads of the second shaft 120, thereby preventing "float" inthe blade. The thimble 130 typically has a scale (not shown) on theouter surface that provides an indication of the relative distancebetween the blade 106 and footplate 114.

The shaft 108 has a magnetic core 134 that is coupled to a transformersleeve 136. The transformer sleeve 136 is coupled to the visual displaysystem 20 by wires 138 that extend through the knife 100. The wires 138are typically introduced through an end cap 140 that is attached to thesecond shaft 120. The sleeve 136 typically contains three coils whichtogether with the magnetic core 134 comprise a linear variabledifferential transformer ("LVDT"). Such a sleeve is sold by LucasSchaevitz under the designation "MP Series". The LVDT is anelectromechanical device that produces an electrical output that islinearly proportional to the linear displacement of the magnetic core134. Use of an LVDT in a surgical knife is disclosed in U.S. Pat. No.4,750,489 issued to Berkman, et al, which is hereby incorporated byreference.

The coils of the sleeve 136 are connected to the main display unit 20which receives the electrical output of the LVDT and converts the signalinto a visual alphanumeric display. The unit also has the buttons 41 and43 which allows the user to zero and recalibrate the display,respectively. Such a unit is also sold by Lucas Schaevitz under the "MPSeries" designation. Although an LVDT and display unit are shown anddescribed, it is to be understood that the knife can be zeroed and setwithout the LVDT or display unit by using the markings on the sleeve 42.

FIG. 15 shows an alternate embodiment of the knife. Instead of the LVDT,the knife 200 may contain an optical encoder 202. The optical encoder202 typically has a plurality of markings coupled to the shaft 108 andan optical sensor unit attached to the body 102 of the knife. Theoptical encoder 202 produces an electrical output that corresponds tothe rotational movement of the shaft 108. The electrical output of theencoder 202 is interpreted by the visual display system 20 to produce avisual alphanumeric display, which corresponds to the distance betweenthe tip of the blade and the footplate of the knife. An optical encoderthat provides an output in response to rotational movement is sold byHewlett-Packard under the designations HEDS 5500/5540 and 5600/5640.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

What is claimed is:
 1. An apparatus allows a viewer to calibrate asurgical knife that has a body, and a blade which has a tip that canmove relative to a foot plate comprising;a calibration housing which hasa holder and a first sleeve, said calibration housing having an innercavity that can receive the blade and at least a portion of the body,said first sleeve having a first marking and at least a segment which istransparent so that the viewer can view said first marking and the tipof the blade when the knife is inserted into the calibration housing,said sleeve further having an aligning surface which engages the knifeand aligns the foot plate with said first marking.
 2. The apparatus asrecited in claim 1, wherein said first sleeve is detachably connected tosaid holder.
 3. The apparatus as recited in claim 1, further comprisinga second sleeve adapted to be inserted into said inner cavity toseparate the blade from said calibration housing.
 4. The apparatus asrecited in claim 1, wherein said transparent segment of said firstsleeve has a flat surface.
 5. The apparatus as recited in claim 1,wherein said first sleeve has a first wall separated from a second wallby said inner cavity such that the blade can be inserted between saidfirst and second walls, said first sleeve having a first marking on saidfirst wall that is parallel with a first marking on said second wallwhen viewed from a point that is perpendicular to said first markings.6. The apparatus as recited in claim 1, wherein said first sleeve has asecond marking located a predetermined distance from said first marking.7. The apparatus as recited in claim 5, wherein said first sleeve has arectangular cross-section.
 8. The apparatus as recited in claim 1,further comprising optic means operatively connected to said calibrationhousing for magnifying said first marking and the blade.
 9. Theapparatus as recited in claim 8, further comprising arm means for movingsaid calibration housing relative to said optic means.
 10. The apparatusas recited in claim 8, wherein said optic means includes a microscopethat has two eyepieces and a lens located between said microscope andsaid first sleeve to further magnify said first marking and the blade.11. The apparatus as recited in claim 10, wherein said optic meansincludes light bending means for bending light that is transmitted intosaid eyepieces from said first sleeve.
 12. The apparatus as recited inclaim 11, wherein said optic means includes deflection means fordeflecting light from an object into one of said eyepieces.
 13. Theapparatus as recited in claim 12, wherein said object is a visualreadout that indicates a position of the tip of the blade relative tothe body.
 14. The apparatus as recited in claim 1, wherein said firstsleeve has a plurality of markings which define a scale, said scalebeing adapted to provide the viewer with an indication of the positionof the blade tip relative to the body.
 15. The apparatus as recited inclaim 14, wherein said first sleeve has a plurality of marking whichdefine a scale, said scale being adapted to provide the viewer with anindication of the position of said blade tip relative to said body. 16.A system that can be used by a viewer to calibrate a surgical knife,comprising;a surgical knife that has a body and a blade with a tip thatcan move relative to a foot plate; sensing means for sensing theposition of said tip relative to said body and providing an outputsignal thereof; visual display means coupled to said sensing means forreceiving said output signal and providing a visual readout of theposition of said blade tip relative to said body, the visual displayfurther having input means to allow the viewer to manually set a zero onthe visual readout; and, a calibration housing which has a holder and afirst sleeve, said calibration housing having an inner cavity that canreceive said blade and at least a portion of said body, said firstsleeve having a first marking and at least a segment which istransparent so that the viewer can view said first marking and said tipof said blade when said surgical knife is inserted into said calibrationhousing, said sleeve further having an aligning surface which engagesthe knife and aligns the foot plate with said first marking.
 17. Thesystem as recited in claim 16, wherein said sensor means includes amagnetic core member attached to said blade and coil means coupled tosaid magnetic core member for detecting the position of said tiprelative to said body.
 18. The system as recited in claim 17, whereinsaid coil means is attached to said calibration housing.
 19. The systemas recited in claim 16, wherein said surgical knife has movement meansfor allowing the viewer to manually move said blade relative to saidbody.
 20. The system as recited in claim 17, wherein said movement meanscan be coupled to an eccentric planetary gear operatively connected tosaid calibration housing, said eccentric planetary gear being coupled toa pinion gear which is driven by an electric motor.
 21. The system asrecited in claim 16, wherein said surgical knife has a first gear with athreaded bore and an external thread that engages an internal thread ofsaid body, said blade being connected to a shaft which has a threadedshank that engages said threaded bore, said surgical knife furtherhaving a thimble connected to said first gear such that rotation of saidthimble rotates said first gear and moves said blade along alongitudinal axis of said surgical knife.
 22. The system as recited inclaim 16, wherein said first sleeve is detachably connected to saidholder.
 23. The system as recited in claim 16, further comprising asecond sleeve adapted to be inserted into said inner cavity to separatesaid blade from said calibration housing.
 24. The apparatus as recitedin claim 22, wherein said transparent segment of said first sleeve has aflat surface.
 25. The apparatus as recited in claim 24, wherein saidfirst sleeve has a first wall separated from a second wall by said innercavity such that said blade can be inserted between said first andsecond walls, said first sleeve having a first marking on said firstwall that is parallel with a first marking on said second wall whenviewed from a point that is perpendicular to said first markings. 26.The apparatus as recited in claim 25, wherein said first sleeve has asecond marking located a predetermined distance from said first marking.27. The apparatus as recited in claim 16, further comprising optic meansoperatively connected to said calibration housing for magnifying saidfirst marking and said blade.
 28. The apparatus as recited in claim 27,further comprising arm means for moving said calibration housingrelative to said optic means.
 29. The apparatus as recited in claim 27,wherein said optic means includes a microscope that has two eyepiecesand a lens located between said microscope and said first sleeve tofurther magnify said first sleeve and said blade.
 30. The apparatus asrecited in claim 29, wherein said optic means includes light bendingmeans for bending light that is transmitted into said eyepieces fromsaid first sleeve.
 31. The apparatus as recited in claim 30, whereinsaid optic means includes deflection means for deflecting light fromsaid visual display into one of said eyepieces, so that the viewer cansee said visual readout.
 32. A system that can be used by a viewer tocalibrate a surgical knife, comprising;a surgical knife that has a bodyand a blade with a tip that can move relative to a foot plate; sensingmeans for sensing the position of said tip relative to said body andproviding an output signal thereof; visual display means coupled to saidsensing means for receiving said output signal and providing a visualreadout of the position of said blade tip relative to said body, thevisual display further having input means to allow the viewer tomanually set a zero on the visual readout; a calibration housing whichhas a holder and a first sleeve, said calibration housing having aninner cavity that can receive said blade and at least a portion of saidbody, said first sleeve having a first marking and at least a segmentwhich is transparent so that the viewer can view said first marking andsaid tip of said blade when said surgical knife is inserted into saidinner cavity, said sleeve further having an aligning surface whichengages the knife and aligns the foot plate with said first marking; amicroscope that magnifies said first sleeve, said microscope having atleast one eyepiece that allows the viewer to view said blade tip andsaid sleeve; and, an arm attached to said calibration housing andconnected to said microscope so that the viewer can move said firstsleeve under said microscope.
 33. The system as recited in claim 32,wherein said first sleeve is detachably connected to said holder. 34.The system as recited in claim 33, wherein said transparent segment ofsaid first sleeve has a flat surface.
 35. The apparatus as recited inclaim 34, wherein said first sleeve has a first wall separated from asecond wall by said inner cavity such that said blade can be insertedbetween said first and second walls, said first sleeve having a firstmarking on said first wall that is parallel with a first marking on saidsecond wall when viewed from a point that is perpendicular to said firstmarkings.
 36. The apparatus as recited in claim 35, wherein said firstsleeve has a second marking located a predetermined distance from saidfirst marking.
 37. The system as recited in claim 32, further comprisinga second sleeve adapted to be inserted into said inner cavity toseparate said blade from said calibration housing.
 38. The system asrecited in claim 32, wherein said surgical knife has a first gear with athreaded bore and an external thread that engages an internal thread ofsaid body, said blade being connected to a shaft which has a threadedshank that engages said threaded bore, said surgical knife furtherhaving a thimble connected to said first gear such that rotation of saidthimble rotates said first gear and moves said blade along alongitudinal axis of said surgical knife.
 39. The system as recited inclaim 38, wherein said sensing means includes a magnetic core memberattached to said first gear and coil means coupled to said magnetic coremember for detecting the position of said tip relative to said body. 40.The system as recited in claim 39, wherein said coil means is attachedto said calibration housing.
 41. The system as recited in claim 38,wherein said thimble has an external thread that is coupled to aneccentric planetary gear, said eccentric planetary gear being coupled toa pinion gear that is driven by an electric motor.
 42. The system asrecited in claim 32, further comprising a lens attached to said arm andlocated between said microscope and said first sleeve for furthermagnifying said first marking and said blade tip.
 43. The system asrecited in claim 42, further comprising light bending means attached tosaid arm for bending light that is transmitted into said eyepiece fromsaid first sleeve.
 44. The system as recited in claim 42, furthercomprising deflection means attached to said arm for deflecting lightfrom said visual display into said eyepieces so that the viewer can seesaid visual readout.
 45. The apparatus as recited in claim 35, whereinsaid first sleeve has a plurality of markings that define a scale, saidscale being adapted to provide the viewer with an indication of theposition of said blade tip relative to said body.
 46. A method forzeroing a surgical knife that has a blade tip which can move relative toa footplate, the surgical knife being coupled to a visual display bysensor means which detect the position of the blade tip relative to thefootplate and provide an output signal that is received by the visualdisplay and converted into a visual readout that provides a viewer anindication of the relative position of the blade tip and the footplate,the visual display further having input means for allowing the viewer tozero the visual display, comprising the steps of;a) inserting thesurgical knife into a calibration housing which has a sleeve that has afirst marking, said surgical knife being inserted until the footplate isparallel with said first marking, said calibration housing being coupledto an optical system that magnifies said sleeve so that the viewer cansee the footplate, the blade tip and said first marking; b) moving theblade tip until the blade tip is parallel to said first marking; and, c)inputting a zero into the visual display.
 47. The method as recited inclaim 46, further including a method for calibrating the visual displaywherein said sleeve has a second marking located a predetermineddistance from said first marking and further comprising the steps of;d)moving the blade tip until the blade tip is parallel with said secondmarking; e) viewing the visual display to determine whether the visualreadout displays said predetermined distance between said first andsecond markings.
 48. The method as recited in claim 47, furthercomprising the step (f) of inputting said distance into the visualdisplay.
 49. A method for setting a surgical knife that has a blade tipwhich can move relative to a footplate, comprising the steps of;a)inserting the surgical knife into a calibration housing which has asleeve that has a plurality of markings, said markings each beingseparated a predetermined distance such that said markings define ascale, said calibration housing being coupled to an optical system thatmagnifies said sleeve so that the viewer can see the footplate, theblade tip and said markings; and, b) moving the blade tip until theblade tip is parallel with a predetermined marking.